toadatrix May 26, 2004
--- In guide-user@yahoogroups.com, Bill J Gray <pluto@p...> wrote:
> Hi John,
>
> The problem is in Guide's table for Delta-T. It currently
looks like
> this, with data up to 1 January 2004 coming from actual
observations
> and the 2006, 2008, and 2010 data coming from USNO predictions
made
> in 2001:
>
> Year Delta-T
>
> 1990 56.8554
> 1992 58.3093
> 1994 59.9847
> 1996 61.6287
> 1998 62.9659
> 2000 63.8285268
> 2002 64.2998152
> 2004 64.5736400
> 2006 67.6 +/- 2.
> 2008 68.9 +/- 3.
> 2010 70.2 +/- 5.
>
> The problem comes from those predictions, made at a time when
> Delta-T was steadily increasing at about a second a year. USNO has
> discontinued these predictions, so I can't replace those values
with
> something more current. For dates near the present, Guide is
> interpolating between the (perfectly reasonable) 2004 value and the
> (now not-so-reasonable looking) 2006 value. The end result is:
Guide
> thinks Delta-T is growing pretty briskly, so it's getting out of
sync
> with UTC.
>
> Fortunately, there is a way around this problem. We can tell
Guide
> what formula to use for Delta-T, as described at
>
> http://www.projectpluto.com/update7.htm#delta_t_def
>
> What I'd suggest doing is to tell Guide that, for dates from
2004
> on, Delta-T is increasing linearly at its current slow rate. That
> means hitting Alt-J and entering the following:
>
> DELTA_T=2004,2100:64.574,18.62,0
>
> You will still eventually see discrepancies between UT and UTC
that
> are greater than .9 seconds. But attempting to evade such
discrepancies
> does not strike me as especially realistic. It would be more of a
> cosmetic fix than anything resembling "what will actually happen".
>
> More specifically, if someone tells you they used, say,
> Delta-T=64.9 seconds, and you want to replicate their results,
> you could "fix" Guide's Delta-T at a constant 64.9 seconds with
>
> DELTA_T=2004,2100:64.9,0,0
>
> Incidentally, "UT" in Guide means "UT1". (UT0 is time
corrected for
> the observer location on the earth, with the effects of polar
motion
> included. UT1 is a "standardized" sort of UT for a non-wandering
pole.
> UT2 is essentially obsolete. The difference between UT0 and UT1
will
> become crucial if Guide is to be accurate at the level of a
> few centimeters, instead of at the level of a dozen meters. So
far,
> the only Guide user with an interest in that level of accuracy has
been
> its author. I'd like to be able to replicate the laser ranging
> observations made of the mirrors left by the Apollo missions.)
>
> -- Bill